I. Field of the Invention
The present invention relates generally to communication systems and particularly to power control in a radiotelephone system.
II. Description of the Related Art
The Federal Communications Commission (FCC) governs the use of the radio frequency (RF) spectrum. The FCC allocates certain bandwidths within the RF spectrum for specific uses. A user of an allocated bandwidth of the RF spectrum must take measures to ensure that the radiated emissions inside and outside of that bandwidth are maintained within acceptable levels to avoid interfering with other users operating in the same or other bandwidths. These levels are governed by both the FCC and the particular user groups of the bandwidth.
The 800 MHz cellular radiotelephone system operates its forward link, the cell to radiotelephone transmission, in the bandwidth of 869.01 MHz to 893.97 MHz. The reverse link, the radiotelephone to cell transmission, is in the bandwidth of 824.01 MHz to 848.97 MHz. The forward and reverse link bandwidths are split up into channels, each channel occupying a 30 kHz bandwidth. A particular user of the cellular system may operate on one or several of these channels at the same time.
There are several different techniques of modulation that can be used in the cellular radiotelephone system. Two examples of modulation techniques are frequency division multiple access (FDMA) and code division multiple access (CDMA).
The FDMA modulation technique generates signals that occupy one channel at a time while the CDMA modulation technique generates signals that occupy several channels. Both of these techniques must control their return link radiated emissions to within acceptable limits inside and outside of the assigned channel or channels. For maximum system performance, users of the CDMA technique must carefully control the level of radiated power inside the channels in which they are operating.
A CDMA receiver that receives digitally modulated information signals generally includes a variable gain amplifier with a gain adjusted by a control signal. The process of adjusting the gain of a received signal using a control signal is called automatic gain control (AGC). Typically, in digital receivers, the AGC process involves measurement of an output signal power of the variable gain amplifier.
The measured value is compared with a reference value, representing the desired signal power, to generate an error signal. The error signal is then used to control the variable amplifier gain so as to adjust the signal strength to coincide with the desired signal power.
To effect digital demodulation with an optimal signal to noise ratio, AGC is used to hold the magnitude of the baseband waveforms close to the full dynamic range of the baseband analog to digital converters. This generally requires that the AGC be provided over the full dynamic range of the received signal power.
FIG. 1 shows a typical prior art cellular radiotelephone. This radiotelephone is comprised of a transmit section 102 and a receive section 103. In the transmit section 102, a microphone 110 picks up a voice signal and changes it into an analog signal that is encoded and modulated 115. The modulated signal, T.sub.x, is at an intermediate frequency. T.sub.x is input to an automatic gain control (AGC) amplifier 120. For CDMA radiotelephones the transmit AGC 120 is controlled by a combination 125 of the power level of the received signal, also known as open loop power control, and transmit power commands from the cell 130, also known as closed loop power control. Both open and closed loop power control are discussed in further detail in U.S. Pat. No. 5,056,109 issued to Gilhousen et al.
The signal from the AGC amplifier is input to a power amplifier 101. The amplified signal from the power amplifier 101 is input to a duplexer 145 that couples the signal to an antenna 150 for transmission over the channel.
In the receive section 103, a signal received by the antenna 150 is coupled to the receive section 103 by the duplexer 145. The received signal is input to a low noise amplifier (LNA) 155. The amplified signal from the LNA 155 is then input to the receive AGC 160. This AGC 160 is controlled 165 by the power level of the received signal. The signal from the receive AGC 160 is demodulated and decoded 170 before being transmitted as an analog voice signal by the speaker 175.
In CDMA based radiotelephones, it is possible to drive the power amplifier 101 in the transmitter beyond a power level where acceptable out-of-channel radiated emissions can be maintained. This is primarily due to the increased output distortion levels of the power amplifier 101 at high output powers. Also, driving the power amplifier 101 beyond a certain level can cause interference internal to the radiotelephone.
In CDMA based radiotelephones, the proper transmitted output power is determined by "open loop" estimates made by the phone, and by "closed loop" correction commands from the base station. The phone makes its open loop estimate by measuring the power received from the base station and assuming a symmetric channel, which is to say, for each dB that the receive signal is below a certain reference level, the phone will transit above another reference level by that same number of dB. This is generally accomplished by employing Automatic Gain Control amplifiers for both the receive and transmit paths that are "dB linear", and coupling their control signal to a common control mechanism which functions to bring the receive signal to the desired setpoint.
In theory, such an arrangement will produce the desired transmit output power. However, maintaining the proper on-channel output power can be difficult due to several undesirable effects in the radiotelephone hardware. For example, the CDMA based radiotelephone must implement a transmit power control scheme that operates over a very wide dynamic range, 80 dB to 90 dB. Any deviation from "dB Linear" in the transmit or receive AGC amplifiers will cause errors in the open loop output power level. Also, any gain variation (such as temperature or frequency induced) in the radio that does not affect the transmit and receive sections equally will cause errors in the power control performance.
Nonlinear errors also occur when lower quality components are used to reduce production costs or when lower power components are used to reduce power consumption. There is a resulting need for linear automatic gain control in a radiotelephone.